Myocardial ischemia refers to a condition in which the blood supply to a region of myocardium (i.e., heart muscle) becomes so compromised that the region is not supplied with adequate oxygen for oxidative metabolism. Ischemia, as opposed to hypoxia without any reduction in perfusion, is also accompanied by reduced removal of metabolic by-products. The heart is an aerobic organ that generates energy almost exclusively from the oxidation of substrates with oxygen delivered by the blood. It can develop only a small oxygen debt and is therefore extremely sensitive to disruptions in blood supply. Myocardial ischemia occurs when there is an imbalance between oxygen supply and demand as a result of increased myocardial oxygen demand, reduced myocardial oxygen supply, or both. Myocardial ischemia causes many patients to experience chest pain or discomfort, referred to as angina pectoris. Angina pectoris can serve as a useful warning of insufficient myocardial perfusion that can lead to the more serious situation such as a heart attack or cardiac arrhythmia.
Coronary artery disease (CAD) occurs when the coronary arteries that supply blood to the myocardium become hardened and narrowed due to the buildup of atherosclerotic plaque. An atherosclerotic plaque is the site of an inflammatory reaction within the wall of an artery and is made up of a core containing lipid and inflammatory cells surrounded by a connective tissue capsule. A myocardial infarction (MI), or heart attack, occurs when atherosclerotic plaque within a coronary artery ruptures and leads to the clotting of blood (thrombosis) within the artery by exposing the highly thrombogenic lipid core of the plaque to the blood. The complete or nearly complete obstruction to coronary blood flow can damage a substantial area of heart tissue and cause sudden death, usually due to an abnormal heart rhythm that prevents effective pumping.
In the presence of coronary obstruction due to CAD, an increase in myocardial oxygen requirements brought about by, for example, physical exertion or emotional distress, can cause a temporary imbalance in oxygen supply and demand. Such demand ischemia can cause what is called exertional anginal or chronic stable angina. In other situations, an imbalance can occur acutely due to a sudden reduction in blood flow, sometimes referred to as supply ischemia. An acute blood flow disruption may be secondary to a coronary vasospasm, causing what is called unstable angina. As noted above, an acute blood flow disruption can also result from coronary thrombosis, causing an MI. Myocardial ischemia often results from both an increase in oxygen demand and a reduction in supply.